1. Field of the Invention
This invention relates to analyte test instruments that perform electrochemical assays on biological samples. More particularly, the invention relates to analyte test instruments that can perform electrochemical assays by using different modes of operation.
2. Discussion of the Art
Electrochemical assays for determining the concentrations of analytes in samples comprising complex mixtures of liquids have been developed. Such electrochemical assays can be performed with test strips, i.e., biosensors in the form of strips. Test strips can function in an invasive manner (i.e., as probes that come into contact with a body fluid, such as whole blood or subcutaneous fluid). Test strips can function in a non-invasive manner (i.e., as strips that come into contact with blood withdrawn by a syringe or a lancing device). In particular, test strips for biomedical applications (e.g., whole blood analyses) have been developed for the determination of glucose levels in biological samples.
An analyte test instrument is an instrument can be used to perform electrochemical assays to determine the concentration of an analyte (e.g., glucose) in a biological sample (e.g., blood). To operate such an instrument, a user inserts a test strip into a test port in the instrument. The instrument displays a “ready” indication to the user and allows sufficient time for the user to deposit a biological sample on the test strip. When a sufficient quantity of the sample reaches the working electrode of the test strip, an electrochemical reaction occurs. The reaction produces an electrical response, such as a change in current. The electrical response is detectable by the analyte test instrument. The analyte test instrument converts the detected signal into data that corresponds to information relating to the analyte and displays the information to the user. The instrument may be able to store a series of such measurements and provide the stored information to the user via a display or to an external processor via a data link.
All commercially available electrochemical assays employing test strips for determining the concentration of glucose employ test strips having two electrodes. See, for example, WO 99/19507, incorporated herein by reference, which describes a test strip having two electrodes. In a test strip having two electrodes, the test strip has (1) a working electrode and (2) a dual-purpose reference/counter electrode. The reaction that takes place at the working electrode is the reaction that is required to be monitored and controlled. The second electrode is called a dual-purpose reference/counter electrode because this electrode acts as a reference electrode as well as a counter electrode. No current passes through an ideal reference electrode, and such an electrode maintains a steady potential; current does pass through a dual-purpose reference/counter electrode, and thus, the dual-purpose reference/counter electrode does not maintain a steady potential during the measurement. At low currents and/or at short durations of time for measurement, the shift in potential is small enough such that the response at the working electrode is not significantly affected, and hence the dual-purpose reference/counter electrode is designated a dual-purpose reference/counter electrode. The dual-purpose reference/counter electrode continues to carry out the function of a counter electrode; however, in this case, the potential that is applied between the dual-purpose reference/counter electrode and the working electrode cannot be altered to compensate for changes in potential at the working electrode.
Electrochemical assays employing test strips having three electrodes employ a test strip having (1) a working electrode, (2) a reference electrode, and (3) a counter electrode. See, for example, U.S. Ser. No. 10/062,313, filed Feb. 1, 2002, incorporated herein by reference. As in the test strip having two electrodes, the reaction that takes place at the working electrode is the reaction that is required to be monitored and controlled. The functions of the reference electrode and the counter electrode are to ensure that the working electrode actually experiences the conditions desired, i.e. the correct potential intended to be applied. The function of the reference electrode is to measure the potential at the interface of the working electrode and the sample as accurately as possible. In an ideal situation, no current passes through the reference electrode. The function of the counter electrode is to ensure that the correct potential difference between the reference electrode and the working electrode is being applied. The potential difference between the working electrode and the reference electrode is assumed to be the same as the desired potential at the working electrode. If the potential measured at the working electrode is not the potential desired at the working electrode, the potential that is applied between the counter electrode and the working electrode is altered accordingly, i.e., the potential is either increased or decreased. The reaction at the counter electrode, as measured by the current, is also equal and opposite to the charge transfer reaction, as measured by the current, occurring at the working electrode, i.e., if an oxidation reaction is occurring at the working electrode then a reduction reaction will take place at the counter electrode, thereby allowing the sample to remain electrically neutral.
An analyte test instrument designed for test strips having two electrodes could not be used if an assay employing a test strip having three electrodes needs to be performed. The user would have to use a separate analyte test instrument. If the user wanted to perform a set of assays that required strips having two electrodes and a set of assays that required strips having three electrodes, these assays could not be performed on the same analyte test instrument.
An analyte test instrument for electrochemical assays often requires the user to calibrate the instrument for each batch of test strips. U.S. Pat. No. 5,366,609, incorporated herein by reference, describes a calibration technique that requires a read-only-memory (ROM) key for operation and calibration of an analyte test instrument. A ROM key is inserted into a port (i.e., the ROM key port) that is distinct from the port for a test strip (i.e., the test port). A test strip is inserted into the test port after the ROM key is inserted into the ROM key port. The ROM key must remain in the ROM key port during both the calibration and the operation of the instrument. The ROM key contains specific data, including algorithms, for carrying out procedures for determining the concentration of an analyte in a biological sample applied to one of a batch of test strips associated with the ROM key. The data stored in the ROM key include measurement delay times, incubation times, the number of measurements to be taken during a measurement period, various thresholds against which voltage levels can be compared, values of excitation voltage levels applied to the strip during a test procedure, glucose value conversion factors, and a variety of failsafe test threshold values. In addition, the ROM key can contain some or all of the code for the microprocessor that controls the performing of the assay. A microprocessor in the analyte test instrument uses the algorithms, the conversion factors, and the code provided by the ROM key as needed.
U.S. Pat. No. 6,377,894, incorporated herein by reference, describes an instrument requiring a ROM key for operation and calibration of the instrument. The ROM key is inserted into the test port of the instrument and data is downloaded from the ROM key by the instrument and stored in the memory of the instrument. The ROM key contains data needed for carrying out procedures for determining the concentration of an analyte in a biological sample applied to a test strip. The ROM key is removed so that test strips can be inserted into the test port to perform assays. Different ROM keys can be inserted into the instrument to provide data for the testing of different analytes on the same instrument. The instrument can communicate with the ROM key to determine the analyte for which the ROM key contains information. Calibration information can be stored in different locations in the memory of the instrument for each analyte the instrument is capable of testing. When a test strip is inserted into the test port, the instrument has the ability to recognize which analyte is being tested. The microprocessor in the instrument then recalls the instructions for carrying out procedures for determining the concentration of that analyte, and the instrument then performs the appropriate test.
The aforementioned patents do not describe how the electrical circuitry of the instrument can be reconfigured so that analytical tests that require different circuit configurations can be performed on the same instrument. The aforementioned patents do not describe how stored information relating to the configuration of the electrical circuitry of the instrument can be modified when an assay for a specific analyte needs to be modified. The aforementioned patents do not describe how stored information can be used to reconfigure the electrical circuitry of the instrument while a test, strip is being used. Accordingly, it would be desirable to provide an analyte test instrument that addresses the foregoing deficiencies.